1. Field of the Invention
The present invention relates to a rolling bearing for bearing a pulley shaft of a belt continuously variable transmission for a vehicle.
2. Description of the Related Art
A belt continuously variable transmission for vehicle has a mechanism for continuously changing the radius of belt-driven pulley as a variable speed mechanism for automatic transmission. For example, as shown in FIG. 6, an input shaft (drive shaft) 5 and an output shaft (driven shaft) 6 which are disposed in parallel to each other are provided with pulleys 7 and 8, respectively. A metallic belt 9 is wound round these pulleys. This belt 9 comprises two lines of ring composed of a laminate of about 10 sheets of thin plates having a thickness of about 0.2 mm and a number of thin friction pieces 92 (having a thickness of about 2 mm) attached thereto. The pushing force developed when these friction pieces 92 push each other causes the power to be transmitted.
A driving force is transmitted from the input shaft pulley (primary pulley) to the output shaft pulley (secondary pulley) 8 via this belt 9. Both the pulleys 7, 8 are composed of fixed conical plates 71, 81 fixed to the shafts 5, 6 and movable conical plates 72, 82 capable of moving in the axial direction by a hydraulic mechanism, respectively. The two conical plates form a V-shaped pulley groove.
By moving the movable plates 72, 82 of the pulleys 7, 8 in the axial direction to change the width of the groove and changing the position at which the belt 9 comes in contact with the pulleys 7, 8, the gear ratio can be continuously changed. For example, by reducing the width of the groove of the input shaft pulley while increasing the width of the output shaft pulley, the effective radius of rotation of the input shaft pulley is reduced while the effective radius of rotation of the output shaft pulley is increased, obtaining a great gear ratio.
Shafts (pulley shaft) 71a, 81a integrated to the fixed conical plates 71, 81 of the pulleys 7, 8 are born by radial ball bearings 11, 12, respectively. These pulley shafts 71a, 81a are each subject to thrust load as a reaction force when they transmit the shaft output to the following stage. Therefore, it is necessary to prevent the radial ball bearings 11, 12 from being dislocated in the axial direction by this thrust load to cause the center thereof to deviate from the input shaft pulley to the output shaft pulley (so-called “center dislocation”). As the aforementioned center dislocation increases, the belt 9 meanders to cause the ring 91 and the friction piece 92 to make inappropriate contact with each other occasionally to damage. Alternatively, the ball bearings 11, 12 can undergo slippage to generate a large amount of heat.
As a measure against this problem, JP-B-8-30526 proposes that a ball bearing for bearing a pulley shaft be provided with a ratio (R/D) of radius (R) of curvature of inner and outer ring raceways to ball diameter (D) of smaller than an ordinary standard predetermined value (0.53) (50.1 to 50.9% for inner ring and 50.1 to 51.9% for outer ring). The smaller this ratio (R/D) is, the more difficultly can move the ball bearing in the axial direction under thrust load and hence the more difficultly can occur center dislocation between the pulleys.
JP-A-10-292859 proposes the use of a four-point contact ball bearing having retained austenite in the inner ring and balls in an amount of not greater than 5%. In some detail, by predetermining the retained austenite content to not greater than 5%, the dimensional change due to the decomposition of retained austenite during heat generation is lessened.
On the other hand, as a lubricant for belt continuously variable transmission, a traction oil (lubricant containing a special abrasion adjustor having a traction coefficient of not smaller than 0.09 and a viscosity of not smaller than 30.8 cst (30.8×10−5 m2/s) at 40° C.) is applied to the aforementioned ball bearing for bearing pulley shaft as well to allow torque converter, gear mechanism, hydraulic mechanism, wet clutch, etc. to operate smoothly for power transmission.
When the ratio (R/D) is reduced as in JP-B-8-30526, the contact area of the raceway with the balls increases, and the surface pressure is reduced. Thus, it can be expected that the bearing life is prolonged.
However, it was found that a radial ball bearing lubricated with a traction oil can difficultly be provided with an expected prolonged life merely by reducing the surface pressure. This is presumably because the lubricant is exfoliated in different manners from traction oil to mineral oil.
In other words, in the case where the lubricant is a mineral oil, the starting point of flaking is located at the center of the width of the raceway as shown in FIG. 7A. On the contrary, in the case where the lubricant is a traction oil, the starting point of flaking is mostly located at a site apart from the center of the width of the raceway as shown in FIG. 7B. Since the difference V in rotary speed between the raceway and the balls is a great factor, it is thought that the bearing life is somewhat affected not only by the surface pressure developed by the raceway and the balls but also by the difference V in rotary speed. Further, the reduction of ratio (R/D) leads to ease of the balls in coming in contact with sites apart from the center of the width of the raceway.
Accordingly, a radial ball bearing for bearing the pulley shaft of a belt continuously variable transmission cannot accomplish both the reduction of center dislocation between the two pulleys and the prolongation of bearing life merely by reducing the aforementioned ratio (R/D).
In addition to this, the short life of rolling bearings for bearing the rotary shaft of pulley of belt continuously variable transmission is presumably attributed to the following mechanism.
In some detail, the rolling bearing for the aforementioned purpose undergoes not only the aforementioned slippage with center dislocation between pulleys but also slippage and vibration due to stick slip friction that occurs on the metallic belt wound round the pulleys. Thus, the lubricant film can be more easily exfoliated. As a result, this bearing is subject to heat generation and surface fatigue due to metallic contact and production of newly produced surface. The newly produced surface then acts as a catalyst to allow hydrocarbons or water content in the lubricant to be decomposed to hydrogen that specifically causes early flaking.